The present invention relates to programmable semiconductor devices, more particularly, to secure electrically programmable fuses (e-fuses) and methods of manufacturing the same.
E-fuses are widely incorporated in circuit designs for a variety of purposes. For example, e-fuses may be incorporated to replace defective cells or circuits, customize a circuit design for a specific application, or encode data such as electronic chip ID or program code. Conventional e-fuse structures are insecure, and as such, the state of one or more fuses may be ascertained by non-destructive radiation imaging techniques such as X-ray imaging. By non-destructively observing the fuse states within a particular circuit design, the state of each e-fuse (e.g. blown or not blown) can be used to breach the security of the design. Known e-fuse states can be used to breach security features of the system since the states can be ascertained without destroying the chip.
Conventional e-fuses take form in a variety of structures. The basic principle of all e-fuses is that an electrically programmable link can be programmed by driving sufficient current through the e-fuse structure, thus raising link temperature until it ruptures. When a sufficient amount of current flows from a first end of the fuse structure, through the fuse link, to a second end of the fuse structure, the fuse link heats up and the resistance of the e-fuse is dramatically changed from a low impedance state to a high impedance state.
A conventional e-fuse structure such as the kind described in commonly-owned, co-pending U.S. patent application Ser. No. 10/904058 entitled PROGRAMMABLE SEMICONDUCTOR DEVICE is now described in accordance with FIGS. 1A and 1B. FIG. 1A illustrates a top-down view of conventional e-fuse structure 100. E-fuse structure 100 comprises fuse link 110 between first end 120 and second end 130. The ends can be symmetric, or alternatively, asymmetric as shown in FIG. 1A, where second end 130 is larger than first end 120. When the ends are asymmetric, the second end is typically referred to as a cathode and the first end as an anode. Reliability of the fuse programming process is improved if the cathode is larger than the anode. Fuse link 110, first end 120 and second end 130 are typically formed from a semiconductor material such as doped polysilicon. The polysilicon can be doped with either n-type or p-type dopants. Optionally, a silicide layer can be formed on the fuse structure to improve the programmability of the fuse. Contacts, such as contacts 125 and 135, are formed on first end 120 and second end 130, respectively, for providing electrically conductive contacts to fuse structure 100.
FIG. 1B illustrates a cross-sectional view of e-fuse structure 100 of FIG. 1A across the width of fuse link 110. The fuse structure is formed on semiconductor substrate 140. Optionally, insulator layer 150 can be formed on semiconductor substrate 140, and the fuse structure can then be formed on insulator layer 150. Fuse link 110 comprises doped polysilicon layer 112 and optional silicide layer 114. Optional silicide layer 114 forms a low resistance portion of the fuse structure. When current flows through the fuse link, polysilicon layer 112 heats up, assisting in the migration of the silicide toward one end of the fuse. The direction of the migration of suicide depends on the current flow direction. Optional insulator layer 150 improves fuse programmability by trapping heat energy between the fuse structure and the substrate, thus maintaining a high temperature at the polysilicon-silicide junction. Insulator region 170 also improves fuse programmability by trapping heat energy within the fuse structure. Non-reactive silicon nitride layer 160 can be optionally formed over fuse link 110.
Conventionally, metal wiring is permitted over the ends of e-fuse structures (e.g. anodes and cathodes), but is strictly prohibited from being placed over the fuse link region itself. Metal is prohibited over the fuse link region because metal wiring layers, in particular thick metal wires, diminish the programmability of e-fuse structures because the wiring layers act as parasitic heat sinks. If the fuse link region is not maintained at a sufficiently elevated temperature during the programming process, the programmability of the fuse may become unreliable. Metal layers tend to sink heat energy away from the fuse region during programming, thus impacting the reliability of the fuse programming process. If the fuse is not maintained at a sufficient temperature throughout the programming process, the fuse link region may not be fully programmed (i.e. some of the fuse link region may remain intact, thus forming a quasi-conductive path between the ends of the fuse). Insulative materials are conventionally formed both above (e.g. SiN) and below (e.g. SiO2) the fuse structure to help maintain an elevated temperature around the fuse during programming by minimizing heat loss through the semiconductor substrate and metal wiring layers.
U.S. Pat. No. 6,166,421 (the '421 Patent), assigned to National Semiconductor Corporation and incorporated herein by reference in its entirety, discusses an alternative e-fuse structure. FIG. 4B of the '421 Patent illustrates a cross-sectional view of e-fuse 400 before programming. The section of polysilicon layer 412 to be programmed, which is formed under insulator layer 414, is not covered by any metal layers or wiring. FIG. 5A illustrates a cross-sectional view of fuse 400 after it has been programmed. The polysilicon layer has migrated into two separate, distinct sections 412A and 412B. The programmed section of the polysilicon link is the portion of polysilicon layer 412 that was formed under insulator 414 in FIG. 4B, but is no longer present because it has been migrated away from this region into cavities 430 of FIG. 4B. Therefore, the fuse state (i.e. programmed or un-programmed) can be determined by subjecting the fuse structure to radiation imaging techniques. For example, X-rays are produced when high energy particles collide with a target and electron energy loss resulting from the collision is manifested as X-rays. When X-rays encounter any form of matter, they are partly transmitted and partly absorbed. X-ray absorption is measured according to the following formula: I=Ioexp−(μ/ρ)(ρx) where I: transmitted beam intensity; Io: incident beam intensity; μ/ρ: mass absorption coefficient; ρ: density; x: distance between source (e.g. emitter) and absorber. Because no X-ray inhibitive material is formed over the programmable region of the fuse, the state of each fuse can be readily determined using non-destructive radiation imaging techniques such as X-ray imaging. The e-fuse states can then be used to breach the security of the circuit design because the design has not been destroyed (e.g. deconstructed), and thus, is subject to observation, operation, and/or reverse-engineering.
Therefore, there exists a need for e-fuse structures having radiation inhibiting properties for preventing non-destructive security breaches, such as by X-ray imaging, but without adversely effecting e-fuse programmability.